1. Field of Invention
The invention generally relates to measuring, in situ, optical parameters using an optical supervisory channel. More particularly, the invention relates to adapting conventional service channel modems, or other devices handling an optical supervisory channel, to measure optical parameters such as chromatic dispersion, link loss, and multi-path interference.
2. Description of Related Art
Fiber-optic communication networks are experiencing rapidly increasing deployment. Especially rapid is the growth of segments that carry multi-gigabit digital data on multiple wavelengths over a single fiber strand. There are two major areas that have seen substantial increases in performance.
The first area is the rapidly increasing bit-rate of a single communication channel. While several years ago 622 Mbps and 2.5 Gbps rates were dominant, current networks are being deployed with 10 GBps data rates, and new network plans using 40 Gbps data rates are being proposed. When modulated onto an optical carrier, the optical spectrum is broadened in linear proportion to the bit-rate. The interaction of the broadened optical spectrum with wavelength-dependant group velocity (chromatic dispersion) in the fiber introduces signal distortions.
The amount of tolerable distortion is inversely proportional to the bit-rate. Thus, the combination of increasing spectral broadening and decreasing distortion tolerance makes the overall propagation penalty proportional to the square of bit-rate. For example, this results in a 10 Gbps signal being 16 times less tolerant to dispersion than 2.5 Gbps signal, while having only 4 times the bit-rate. Dispersion accumulates linearly with propagation distance in the fiber, and typical dispersion-limited propagation distances in standard single-mode fiber (SMF-28 or equivalent) are xcx9c1000 km at 2.5 Gbps, 60 km at 10 Gbps, and only xcx9c4 km at 40 Gbps. Clearly, accurate dispersion measurement is critical for high data rate fiber-optic system deployment. The above linear effect is only sensitive to the total amount of dispersion between the transmitter and receiver.
Second, optical signals experience certain nonlinear interactions when propagating along the optical fiber. These effects, such as self-phase modulation, cross-phase modulation, four-wave mixing, etc also depend on the chromatic dispersion in the optical fiber. In contrast to the linear effects, the nonlinear effects are sensitive to the actual distribution of dispersion values along the optical fiber link (not just total dispersion).
As metropolitan fiber optic networks increase in capacity, they also tend to shift to carrying signals with ever increasing data rates. Further, metropolitan networks tend to have architectures with multiple optical signal add/drop points along the same path. Thus, the specific routes taken by the different WDM signals are varied which leads to variations in the link loss, multi-path interference (MPI) and chromatic dispersion experienced by the different WDM signals as they traverse diverse paths. Thus, accurate measurement of optical properties such as link loss, MPI and chromatic dispersion is important particularly measurements for each fiber-optic segment (optical link) between the add/drop points.
Several methods exist for dealing with dispersion measurements. One relies on using a stand-alone dispersion measurement test-set to measure each fiber-optic link before optical data signals are connected to it. Besides requiring a separate instrument, this makes real-time measurements on an installed system impossible. After the system is installed a new measurement cannot be made. Thus, if the original measurement data is lost or inaccurate the system performance will suffer and cannot be corrected.
Another conventional dispersion measuring method relies on utilizing a subcarrier signal that is carried along with the WDM channels between the data transmitter and receiver. This method assumes that such signals are available between all the points that need to be measured, which is not true in most cases. Further, it assumes that the system is fully functional and able to carry data which is also not true in a case of a new installation.
Another problem that is potentially damaging to fiber-optic communication systems is multi-path interference. This problem arises where there are two or more optical reflections along the optical fiber link. Signals arising at these reflections have a delayed arrival time at the receiver with respect to the main signal, giving rise to interferometric noise and additional performance penalties.